Photo Model

class Photo < ActiveRecord::Base
  
  belongs_to :project, :dependent => :destroy
 
  has_attached_file :data, :styles => { :medium => "300x300>", :thumb => "100x100>" }
  validates_attachment_content_type :data, :content_type => 'image/jpeg', :message => "has to be in jpeg format"
  
end

Project Model

class Project < ActiveRecord::Base
  
  attr_accessor :photo_remove
  attr_accessible :name, :description, :url, :photos_attributes
  validates_presence_of :name
  has_many :photos, :dependent => :destroy
 
  accepts_nested_attributes_for :photos, :allow_destroy => true
  
  def photo_remove=(remove)
    self.photo = nil if remove == '1'
  end
end

View(partial)

<% for photo in @project.photos %>
  <p><%= image_tag(photo.data(:thumb)) %></p>
	<label>Delete</label><%= check_box :photo, :photo_remove %>
<% end %>